The present disclosure is related to thermoacoustic devices, and more specifically to a thermoacoustic device employing an acoustic energy converter and electrical impedance network in place of selected portions of an acoustic impedance network.
The Stirling cycle is a well-known 4-part thermodynamic process, typically operating on a gas, to produce work, or conversely to effect heating or refrigeration. The 4 parts are: isothermal expansion, isochoric heat extraction, isothermal compression, and isochoric heat addition. The process is closed, in that the gas remains within the system performing the cycle at all times during the cycle.
A number of devices employing the Stirling cycle are generically referred to as Stirling engines. A typical Stirling engine has a mechanical piston (often two pistons), which responds to the heating/expansion and cooling/contraction of a contained gas as part of the Stirling cycle. The motion of the piston(s) drives a crank from which work can be extracted. An element, typically called a regenerative heat exchanger or regenerator, increases the engine's thermal efficiency. Stirling engines often include a piston and other related moving parts. Devices of this type are often complex, involve seals and pistons, and require regular maintenance.
Thermoacoustic engines are another group of devices utilizing a Stirling thermodynamic cycle. These devices share some fundamental physical properties with Stirling engines, namely a contained gas which approximates a Stirling cycle. However, a thermoacoustic engine differs from a Stirling engine in that a temperature differential amplifies acoustic energy which is extracted for work. Often, there are no pistons or cranks such as typically found in a Stirling engine.
FIG. 5 is a cross-sectional representation of one example 30 of known thermoacoustic engine designs. One embodiment of a thermoacoustic engine known in the art is a hollow, looped, sealed body structure 32 having a regenerator 34 located therein. The regenerator is often simply a metal mesh or matrix. The regenerator is proximate a first heat exchanger 36, generally a “cold” exchanger, at a first end thereof and a second heat exchanger 38, generally a “hot” exchanger, at the opposite end thereof. A third heat exchanger, generally at ambient temperature, may optionally be present. A resonator 40, often in the form of an extension of the hollow body structure is provided. The body structure is filled with a pressurized gas. The temperature differential across the regenerator, i.e., between the hot and cold heat exchangers, subjects the gas to localized heat transfer. Acoustic energy in the form of a pressure wave in the region of the regenerator subjects the gas to local periodic compression and expansion. Under favorable acoustic conditions, the gas effectively undergoes an approximate Stirling cycle in the regenerator.
If the acoustic impedance at the regenerator is low, the pressure oscillations associated with the sound waves are associated with large fluidic displacement velocities. This results in large fluidic resistance losses which degrade the efficiency of the device. Therefore, it is desirable to have a large acoustic impedance at the regenerator. Current thermoacoustic heat engines use an acoustic resonator and/or an acoustic feedback network to achieve this large impedance.
Thus, the body structure and resonator form a physical acoustic impedance network such that the pressure wave travels across the regenerator and resonantly feeds back within the body structure. Due to this feedback and the thermal gradient between heat exchangers, the working gas undergoes a Stirling cycle and does work external to the engine. For example, a transducer 42 may be disposed within the body structure or resonator, and a portion of the energy of the pressure wave may be converted to electrical energy by the transducer.
Unlike conventional Stirling engines, most thermoacoustic engines have few if any mechanical moving parts and are therefore very reliable. Furthermore, unlike conventional Stirling engines, the gas within a thermoacoustic engine does not travel significantly within the body structure. Rather, the pressure wave propagates through the gas and the Stirling cycle takes place locally inside the regenerator. Thermoacoustic engines may operate with either primarily traveling- or standing-wave phasing of the acoustic wave in the regenerator. Standing-wave devices are known to be less efficient than traveling-wave devices.
There are numerous other examples of thermoacoustic engines known in the art. U.S. Pat. No. 7,143,586 to Smith et al., U.S. Pat. No. 7,081,699 to Keolian et al., and U.S. Pat. No. 6,578,364 to Corey illustrate several examples, respectively. Each of these US Patents is incorporated herein by reference. However, each of these examples presents its own set of disadvantages. One disadvantage of certain prior art devices is their relatively large size, primarily due to the necessary length of the resonant acoustic impedance network. This size is a disadvantage for many applications, where a compact device is required. And, the topology and/or the relatively large volume of the body results in an increase in acoustic, thermal, and, in some cases, mass-streaming losses. Another disadvantage is that while pistons sealed within cylinders, and crank arms do not always form a part of known thermoacoustic engines, a number of other elements such as drivers for driving motion of portions of the engine body, bellows, and other moving parts (excluding transducers) are present in prior art engines, adding complexity, size, weight, and cost, increasing the number of elements susceptible to failure, and increasing acoustic loss within the system. A still further disadvantage is that the operating frequency of the engine is set by the physical dimensions of the body and resonator, the construction of the regenerator, and the gas used within the body. It cannot be chosen based, for example, on the desire for improved heat transfer at lower frequencies, matching the frequency of the load, etc.